1. Field of the Invention
The present invention relates to a combined linear motor in which a plurality of linear motors are operated in parallel, and in particular to a combined linear motor for driving a moving table of a machine tool.
2. Description of the Related Art
FIG. 4 shows a conventional combined linear motor for driving a moving table of a machine tool. A moving table here is defined as a table of a machine tool, to which a spindle or a tool rest of the machine tool is attached. In the following, a device having three linear motors operating in parallel will be taken as an example of a conventional linear motor.
The conventional device comprises a machine tool part and an NC control part. The machine tool part comprises a base 5 fixedly attached by a plurality of table stoppers 1 for limiting strokes of the moving table, linear guide rails 2, a position detector 3, and linear motor stators 4a, 4b, 4c, and a moving table 9 attached by a linear guide 6, a position detector reader 7, and linear motor movers 8a, 8b, 8c. The NC control part comprises an NC device 10, a servo control section 11, a current distribution operating circuit 12, and current amplifiers 13a, 13b, 13c. In this drawing, only the outline of the moving table 9 is shown for explanation of installation parts beneath the moving table.
In each of the three linear motors employed in the above conventional example, a stator and a mover consist of a magnet and a wire, respectively. Alternatively, other combinations may be possible for a stator and a mover, such as wires and magnets, wires and wires, and so on.
The moving table 9 is moved under movement control, which is achieved in cooperation with the NC device 10, the servo controller 11, the current distribution operating circuit 12, and the current amplifiers 13a, 13b, 13c. The NC device 10 outputs a position command. The servo controller 11 calculates and outputs a current command based on the position command from the NC device 10 and data on a present position supplied from the position detection reader 7. The current distribution operating circuit 12 outputs current distribution commands for the respective motors operating in different phases, based on the data on the present position and the current command. The current amplifiers 13a, 13b, 13c output currents having respective phases based on the current distribution commands, to the movers 8a, 8b, 8c of the linear motors.
Instead of the three linear motors employed in the above conventional art for parallel driving, a fourth or more motor/motors may be provided in response to a demand for a larger thrust. Specifically, addition of a linear motor mover and stator and a current amplifier, which together constitute a fourth linear motor, could easily attain a table moving mechanism with a larger thrust.
In any case, in this conventional device, a single moving table 9 is driven by two or more linear motors, which are driven in parallel at the same time.
When a linear motor is used as a driving source of a moving device of a machine tool, cogging trust of the linear motor may affect machining accuracy. Therefore, as much cogging trust as possible must be suppressed especially in precise or highly accurate machining. Note that "cogging thrust" here corresponds to cogging torque with a rotary motor.
However, generally, suppression of cogging thrust is subject to limits as circular arrangement of movers is not allowed with a linear motor, of which strokes are inevitably limited due to the structure thereof, in other words, due to the existence of both ends of the motor all the time.
In particular, when two or more linear motors are driven in parallel to attain high thrust according to conventional arts described above, phases of cogging thrust of the respective linear motors are made coincident with one another. This may cause a problem of an increase in cogging trust of the linear motors in proportion to the number of linear motors in use.